(function(doc, win) {
    var docEl = doc.documentElement,
      resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
      recalc = function() {
        var clientWidth = docEl.clientWidth
        if (!clientWidth) return
        if (clientWidth >= 640) {
          docEl.style.fontSize = '100px'
        } else {
          docEl.style.fontSize = 100 * (clientWidth / 750) + 'px'
        }
      }
  
    if (!doc.addEventListener) return
    win.addEventListener(resizeEvt, recalc, false)
    doc.addEventListener('DOMContentLoaded', recalc, false)
  })(document, window)